How does the Twinned Spell metamagic option interact with the Draconic Bloodline sorcerer's Elemental Affinity feature? [duplicate]

Question

The sorcerer's Twinned Spell metamagic option states:

When you cast a spell that targets only one creature and doesn’t have a range of self, you can spend a number of sorcery points equal to the spell’s level to target a second creature in range with the same spell (1 sorcery point if the spell is a cantrip).

To be eligible, a spell must be incapable of targeting more than one creature at the spell’s current level.

The Draconic Bloodline sorcerer's 6th-level Elemental Affinity feature (post-errata) states:

Starting at 6th level, when you cast a spell that deals damage of the type associated with your draconic ancestry, you can add your Charisma modifier to one damage roll of that spell.

It's known that Twinned Spell does not create another spell, it just targets another creature with the same spell. For example, you can concentrate on two haste targets because of that. On the other hand, for AoE spells, Elemental Affinity increases the damage for all targets; even if it applies to one damage roll, that damage roll is applied to all targets.

That said, does Twinned Spell use the same damage roll against the two targets, thus allowing Elemental Affinity to apply to both targets? Or does it use different damage rolls, and thus Elemental Affinity applies only to one target?

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The description of damage rolls in the rules says

If a spell or other effect deals damage to more than one target at the same time, roll the damage once for all of them. For example, when a Wizard casts Fireball or a Cleric casts Flame Strike, the spell’s damage is rolled once for all creatures caught in the blast.

So, the question can be rephrased as: does Twinned Spell make the spell deal damage to both targets at the same time or not?

Answer 1

You make one damage roll

You've quoted the relevant section on spells with multiple targets. Twinned Spell turns the affected spell into such a case, meaning that you roll the damage once. As such, both targets are affected by Elemental Affinity.

when you cast a spell that deals damage of the type associated with your draconic ancestry, you can add your Charisma modifier to one damage roll of that spell.

[emphasis] mine

Answer 2

I have seen the opposite argued convincingly, to wit [italics mine]:

Making an Attack

Whether you’re striking with a melee weapon, firing a weapon at range, or Making an Attack roll as part of a spell, an Attack has a simple structure.

Choose a target. Pick a target within your attack’s range: a creature, an object, or a location.

Determine modifiers. The GM determines whether the target has cover and whether you have advantage or disadvantage against the target. In addition, Spells, Special Abilities, and other Effects can apply penalties or bonuses to your Attack roll.

Resolve the Attack. You make the Attack roll. On a hit, you roll damage, unless the particular Attack has rules that specify otherwise. Some attacks cause Special Effects in addition to or instead of damage.

If there’s ever any question whether something you’re doing counts as an Attack, the rule is simple: if you’re Making an Attack roll, you’re Making an Attack.

Presuming you can't make a single attack roll for two targets - which I think it is safe to presume - the sequence above makes it clear that you make two damage rolls. As a corollary, you are making the attacks sequentially, so presumably can make choices about targets - and who gets the extra damage - on the fly.

EDIT: There's already a StackExchange question that says that there are two damage rolls here: How many attack/damage rolls does a sorcerer make when using the Twinned Spell metamagic option?. (Thanks, Medix2!)